Heat the oil in a saucepan, add the onions and cook over a moderate heat for 5 minutes. Add the tomatoes, garlic, sage and sugar, season with salt and freshly ground black pepper, then cover and cook gently for 30 minutes.
Add the celeriac to the tomato sauce with 1/4 cup of water. Bring to the boil, cover and simmer gently for a further 20 minutes, or until the celeriac is tender.
Transfer the cooked celeriac to a serving dish and garnish with a small sprig of fresh sage.